The highest theoretical DR I can come up with is 260% from the Kroguard(Broguard hits 255%, which is all including Justicar + Volus), but as noted after 140% it's all irrelevant as that's the hard cap.
Although I do recall running tests on DoT damage with the out of cover damage bonus, and it does effect it in real time meaning if you drop into cover after being hit with a Banshee warp you'll take less damage.
Does the damage reduction from reave stack? The Justicar seems to be able to absorb a lot of punishment.
It doesn't stack with itself over multiple applications, but it will stack with the bubble for 80% DR which makes her quite durable especially when in hard cover. The Justicar herself also naturally has slightly higher shields than other Asari.